System and Methods For Nerve Monitoring

ABSTRACT

A system and related methods for performing nerve detection during surgical access using ultrasound testing during surgery.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation U.S. patent application Ser. No.14/881,091 filed Oct. 12, 2015, now pending, which is a continuation ofU.S. patent application Ser. No. 14/063,184 filed Oct. 25, 2013, nowabandoned, which is a continuation of U.S. patent application Ser. No.11/528,981 filed Sep. 27, 2006 (now U.S. Pat. No. 8,568,317), whichclaims priority to U.S. Provisional Patent Application No. 60/721,425filed Sep. 27, 2005, the entire contents of which are hereby expresslyincorporated by reference into this disclosure as if set forth in itsentirety herein.

BACKGROUND OF THE INVENTION Field

The present invention relates generally to a system and related methodsfor performing at least one of bone integrity testing and nervedetection during surgical access using both neurophysiologic testing andultrasound testing during surgery.

Discussion

It has been estimated that somewhere between 50 and 70 million peoplesuffer from chronic back pain in the United States. In most cases,conservative therapies, including, for example, bed rest and physicaltherapy will succeed in alleviating, or at least significantly reducingthe back pain. Still, a significant number of patients are unaided byconservative therapies alone and undergo spinal surgery before findingrelief. The rate at which caregivers and patients opt for surgery alsocontinues to grow as medical technology advances and surgical optionsincrease. In all, approximately 750,000 spine surgeries are performedper year in the United States alone.

When necessary, spine surgery may provide great benefit to the patient,often allowing patients to resume activities long since abandonedbecause of the debilitating pain. Spine surgery, however, is not withoutrisk. Operating on or near the spine generally means operating in closeproximity to delicate neural tissue, such as the spinal cord and nerveroots. Damage to the neural tissue, which may be caused (for example) byinadvertent contact with a surgical instrument and/or implant whileaccessing the spinal target site or inadvertent contact of an implant orsurgical instrument and/or implant before or during pedicle screwplacement. One way to mitigate this risk is to conduct neurophysiologicmonitoring during the procedure and/or recovery period. Neurophysiologicmonitoring generally consists of stimulating neural tissue and analyzingresponses (generally electrical waveforms) generated by the stimulus.While such neurophysiologic monitoring has proved an exceedinglyvaluable tool in efforts to prevent neurological damage during spinesurgery there is still room for further improvements. The presentinvention is directed at such an improvement.

SUMMARY OF THE INVENTION

According to a broad aspect, the present invention includes a surgicalsystem, comprising a surgical instrument having at least one stimulationelectrode for transmitting a stimulation signal for performingneurophysiologic testing during surgery and/or at least one transducerfor transmitting and/or receiving signals for performingultrasound-based testing during surgery. The testing may include, but isnot necessarily limited to, pedicle integrity testing associated withthe use of pedicle screws (e.g. hole formation, preparation, and screwplacement) and surgical access.

BRIEF DESCRIPTION OF THE DRAWINGS

Many advantages of the present invention will be apparent to thoseskilled in the art with a reading of this specification in conjunctionwith the attached drawings, wherein like reference numerals are appliedto like elements and wherein:

FIG. 1 is a perspective view of an exemplary surgical system 10 capableof neurophysiologic assessments together with ultrasound monitoring toaimed to safely access the spine and properly implant pedicle screws forfixation.

FIG. 2 is a block diagram of the surgical system 20 shown in FIG. 1;

FIG. 3 is a graph illustrating an exemplary single pulse stimulationsignal according to one embodiment of the present invention;

FIG. 4 is a graph illustrating an exemplary multipulse stimulationsignal according to one embodiment of the present invention;

FIG. 5 is a graph illustrating an exemplary EMG response to the stimulusof FIG. 3 or 4 according to one embodiment of the present invention;

FIG. 6 is a graph illustrating a plot of peak-to-peak voltage (Vpp) foreach given stimulation current level (I_(Stim)) forming a stimulationcurrent pulse train according to the present invention (otherwise knownas a “recruitment curve”);

FIGS. 7A-7D are graphs illustrating the fundamental steps of a rapidcurrent threshold-hunting algorithm according to one embodiment of thepresent invention;

FIG. 8 is a flowchart illustrating a method by which the algorithm mayomit a stimulation and proceed to the next current according to oneaspect of the present invention;

FIGS. 9A-9C are graphs illustrating use of the threshold huntingalgorithm of FIG. 7 and further omitting stimulations when the likelyresult is already clear from previous data according to one aspect ofthe present invention;

FIG. 10 is an illustration of a stimulation handpiece for couplingsurgical accessories to the neuromonitoring system 10 according to oneembodiment of the present invention;

FIG. 11 an of a screw test probe coupled to the stimulation handpiece ofFIG. 10 illustration according to one embodiment of the presentinvention;

FIG. 12 is an illustration of a stimulation clip coupled to thestimulation handpiece of FIG. 10 according to one embodiment of thepresent invention;

FIG. 13 is a side view of the stimulation probe of FIG. 11 positionedover a stimulation target site and wherein the distal tip of the probemember and the stimulation target site are included in the camera'sfield of view;

FIG. 14 is an exemplary screen view of the Basic Screw Test mode forperforming pedicle integrity assessments according to one embodiment ofthe present invention;

FIG. 15 is an exemplary screen view of the Difference Screw Test modefor performing pedicle integrity assessments according to one embodimentof the present invention;

FIG. 16 is an exemplary screen view of the Dynamic Screw Test mode forperforming pedicle integrity assessments according to one embodiment ofthe present invention;

FIG. 17 is an exemplary screen view of the MaXcess Detection mode fordetecting nerve presence during spinal access according to oneembodiment of the present invention;

FIG. 18 is side view of a tap member coupled to the system for nervemonitoring via an electric coupling device and incorporating anultrasound transducer for intraosteal ultrasound imaging according toone aspect of the present invention;

FIG. 19 is side view of a pedicle access probe/bone awl coupled to thesystem for nerve monitoring via an electric coupling device andincorporating an ultrasound transducer for intraosteal ultrasoundimaging according to one aspect of the present invention;

FIG. 20 is a close up side view of the distal end of the tap member ofFIG. 18 with a cutaway exposing an integrated ultrasound transducer foruse according to one embodiment of the present invention;

FIG. 21 is an overhead view of one embodiment of a transducer for useaccording to one embodiment of the present invention, having 64 elementsarrayed radially for generating 360 degree images;

FIG. 22 is an illustration showing the general makeup of various boneregions comprising the spinal pedicle;

FIG. 23 is an illustration of tap member of FIG. 18 forming a pilot holealong a preferred trajectory utilizing nerve monitoring and ultrasoundimaging to maintain the trajectory;

FIG. 24 is a graphical representation of an ultrasound image showing theposition of a surgical instrument within the interior pedicle duringpedicle integrity testing;

FIG. 25 is a graphical representation of an ultrasound image showing theposition of a surgical access component relative to a nerve during nervedetection;

FIG. 26 is an exemplary screen view of the Basic Screw Test mode forperforming pedicle integrity assessments with concurrent use ofultrasound according to one embodiment of the present invention;

FIG. 27 is an exemplary screen view of the Difference Screw Test modefor performing pedicle integrity assessments with concurrent use ofultrasound according to one embodiment of the present invention;

FIG. 28 is an exemplary screen view of the Dynamic Screw Test mode forperforming pedicle integrity assessments with concurrent use ofultrasound according to one embodiment of the present invention;

FIG. 29 is an exemplary screen view of the MaXcess Detection mode fordetecting nerve presence during spinal access with concurrent use ofultrasound according to one embodiment of the present invention;

DESCRIPTION OF THE PREFERRED EMBODIMENT

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure. The systems disclosed herein boast a variety ofinventive features and components that warrant patent protection, bothindividually and in combination.

The present invention is directed towards enabling safe and reproduciblespinal surgery by aiding in, among other things, access to a target sitein the spine (including but not necessarily limited to a pedicle) andpedicle screw implantation (including but not necessarily limited toformation and preparation of pilot holes and screw placement). To do sothe present invention integrates a host of imaging and neurophysiologicassessment capabilities together in a single, user-friendly and surgeondirected system. FIG. 1 illustrates, by way of example only, a surgicalsystem 10 capable carrying neurophysiologic assessment functionsincluding, but not necessarily limited to, Basic, Difference, andDynamic Screw Tests (pedicle integrity testing), Detection (nerveproximity testing during surgical access) and Free Run EMG (detection ofspontaneous muscle activity, may be conducted in any mode).Simultaneously, the surgical system 10 is capable of performingultrasound imaging to aid in intraoperative guidance of surgicalinstrumentation through bone, and in particular, through the cancellousbone forming the interior region of the pedicle as well as enhancingnerve detection. It is expressly noted that, although described hereinlargely in terms of use in spinal surgery, the surgical system 10 andrelated methods of the present invention are suitable for use in anynumber of additional surgical procedures where neurological impairmentis a concern.

The neuromonitoring system 10 includes a control unit 12, a patientmodule 14, an EMG harness 16, including eight pairs of EMG electrodes 18and a return electrode 22 coupled to the patient module 14, and one ormore of a host of surgical accessories 24 capable of being coupled tothe patient module 14 (preferably via a stimulation handpiece 28 andaccessory cables 26), and a pair of peripheral nerve stimulation (PNS)electrodes (one positive and one negative) 29 also coupled to thepatient module 14. The surgical accessories 24 may include, but are notnecessarily limited to, devices for performing pedicle screw tests (suchas a screw test probe 30, tap member 32, bone awl 34), surgical accesscomponents (such as a K-wire 36, one or more dilating cannula 38, 40, atissue retractor assembly 42), and neural pathology monitoring devices(such as a nerve root retractors 44, 45), any of which may also befitted with one or more ultrasound transducers 55 for imaging ofsurrounding tissue during use. The neuromonitoring system 10accomplishes neuromonitoring by having the control unit 12 and patientmodule 14 cooperate to send stimulation signals to one or morestimulation electrodes or electrode regions on the various surgicalaccessories, while sensors detect muscle activity caused by thestimulation signal.

A block diagram of the neuromonitoring system 10 is shown in FIG. 2, theoperation of which is readily apparent in view of the followingdescription. The control unit 12 includes a touch screen display 46 anda base 48, which collectively contain the essential processingcapabilities for controlling the neuromonitoring system 10. The touchscreen display 26 is preferably equipped with a graphical user interface(GUI) capable of communicating information to the user and receivinginstructions from the user. The base 48 contains computer hardware andsoftware that commands the stimulation and ultrasound sources, receivesdigitized signals and other information from the patient module 14,processes EMG responses, performs ultrasound image processing, anddisplays the processed data to the operator via the display 46. Theprimary functions of the software within the control unit 12 includereceiving user commands via the touch screen display 46, activatingstimulation in the requested mode (Basic Screw Test, Difference ScrewTest, Dynamic Screw Test, MaXcess Detection), processing EMG signal dataaccording to defined algorithms, activating ultrasound signaling,processing ultrasound signal data into viewable images, displayingreceived parameters and processed data, and monitoring system status.

The patient module 14 is connected via a data cable 50 (or optionallyvia wireless communication) to the control unit 12, and contains theelectrical connections to all electrodes, EMG signal conditioningcircuitry, stimulator drive and steering circuitry, ultrasound signalconditioning and receiving circuitry and a digital communicationsinterface to the control unit 12. In use, the control unit 12 issituated outside but close to the surgical field (such as on a cartadjacent the operating table) such that the display 46 is directedtowards the surgeon for easy visualization. The patient module 14 shouldbe located between the patient's legs, or may be affixed to the end ofthe operating table at mid-leg level using a bedrail clamp. The positionselected should be such that the EMG leads surgical accessories 24 canreach their farthest desired location without tension during thesurgical procedure.

The information displayed to the user on the display 46 may include, butis not necessarily limited to, alpha-numeric and/or graphicalinformation regarding any of the requested modes (e.g., Twitch Test,Free-Run EMG, Screw Test (Basic, Difference, Dynamic), Detection, andNerve Retractor), myotome/EMG levels, stimulation levels, paststimulation events, stimulation site images, ultrasound images, etc. . .. . In one embodiment, set forth by way of example only, thisinformation may include at least some of the following components(depending on the active mode) as set forth in Table 1:

TABLE 1 Screen Component Description Spine Image An image of the humanbody/skeleton showing the electrode placement on the body, with labeledchannel number tabs on each side (1-4 on the left and right). Left andright labels will show the patient orientation. The channel number tabsmay be highlighted or colored depending on the specific function beingperformed. Myotome & A label to indicate the Myotome name and LevelNames corresponding Spinal Level(s) associated with the channel ofinterest. Menu A drop down navigation component for toggling betweenfunctions. Display Shows procedure-specific information Area includingstimulation results. Color Enhances stimulation results with a colorIndication display of green, yellow, or red corresponding to therelative safety level determined by the system. Mode Graphics and/orname to indicate the Indicator currently active mode (Detection, BasicScrew Test, Dynamic Screw Test, Difference Screw Test, Free-Run EMG,Twitch Test, Nerve Retractor, MEP, SSEP). In an alternate embodiment,Graphics and/or name may also be displayed to indicate the instrument inuse, such as the dilator, K-wire, retractor blades, screw testinstruments, and associated size information, if applicable, of thecannula, with the numeric size. If no instrument is in use, then noindicator is displayed. Stimulation A graphical stimulation indicatordepicting Bar the present stimulation status (i.e. on or off andstimulation current level) Sequence Shows the last seven stimulationresults Bar and provides for annotation of results. EMG EMG waveformsmay be optionally displayed waveforms on screen along with thestimulation results. Ultrasound Ultrasound images of the tissue,including Image bone, acquired from ultrasound transducers integratedinto or used in cooperation with one or more of the surgicalaccessories.

The neuromonitoring functionality of the neuromonitoring system 10 isbased on assessing the evoked response of the various muscle myotomesmonitored by the system 10 in relation to a stimulation signaltransmitted by the system 10 (via patient module 14). This is best shownin FIG. 3-5, wherein FIG. 5 illustrates the resulting EMG of a monitoredmyotome in response to one of the exemplary single pulse stimulationsignal shown in FIG. 3 and the multiple pulse stimulation signal shownin FIG. 4. The EMG responses provide a quantitative measure of the nervedepolarization caused by the electrical stimulus.

In one embodiment, EMG response monitoring is accomplished via 8 pairsEMG electrodes 18 (placed on the skin over the muscle groups to bemonitored), a common electrode 20 providing a ground reference topre-amplifiers in the patient module 14, and an anode electrode 22providing a return path for the stimulation current. It should beappreciated that any of a variety of known electrodes can be employed,including but not limited to surface pad electrodes and needleelectrodes. It should also be appreciated that EMG electrode placementdepends on a multitude of factors, including for example, the spinallevel and particular nerves at risk and user preference, among others.In one embodiment (set forth by way of example only), an exemplary EMGconfiguration is described for Lumbar surgery in Table 2, Thoracolumbarsurgery in Table 3, and Cervical surgery in Table 4 below:

TABLE 2 Lumbar Color Channel Myotome Nerve Spinal Level Red Right 1Right Vastus Femoral L2, L3, L4 Medialis Orange Right 2 Right TibialisCommon L4, L5 Anterior Peroneal Yellow Right 3 Right Biceps Sciatic L5,S1, S2 Femoris Green Right 4 Right Medial Post Tibial S1, S2 Gastroc.Blue Left 1 Left Vastus Femoral L2, L3, L4 Medialis Violet Left 2 LeftTibialis Common L4, L5 Anterior Peroneal Gray Left 3 Left Biceps SciaticL5, S1, S2 Femoris White Left 4 Left Medial Post Tibial S1, S2 Gastroc.

TABLE 3 Thoracolumbar Color Channel Myotome Nerve Spinal Level Red Right1 Right Abductor Median C6, C7, C8, Pollicis Brevis T1 Orange Right 2Right Vastus Femoral L2, L3, L4 Medialis Yellow Right 3 Right TibialisCommon L4, L5 Anterior Peroneal Green Right 4 Right Abductor Tibial L4,L5, S1 Hallucis Blue Left 1 Left Abductor Median C6, C7, C8, PollicisBrevis T1 Violet Left 2 Left Vastus Femoral L2, L3, L4 Medialis GrayLeft 3 Left Tibialis Common L4, L5 Anterior Peroneal White Left 4 LeftAbductor Tibial L4, L5, S1 Hallucis

TABLE 4 Cervical Color Channel Myotome Nerve Spinal Level Red Right 1Right Deltoid Axilliary C5, C6 Orange Right 2 Right Flexor Carpi MedianC6, C7, C8 Radialis Yellow Right 3 Right Abductor Median C6, C7, C8,Pollicis Brevis T1 Green Right 4 Right Abductor Tibial L4, L5, S1Hallucis Blue Left 1 Left Deltoid Axillary C5, C6 Violet Left 2 LeftFlexor Carpi Median C6, C7, C8 Radialis Gray Left 3 Left Abductor MedianC6, C7, C8, Pollicis Brevis T1 White Left 4 Left Abductor Tibial L4, L5,S1 Hallucis

A basic premise underlying the methods employed by the system 10 formuch of the neurophysiologic monitoring conducted is that neurons andnerves have characteristic threshold current levels (I_(Thresh)) atwhich they will depolarize, resulting in detectable muscle activity.Below this threshold current, stimulation signals will not evoke asignificant EMG response. Each EMG response can be characterized by apeak-to-peak voltage of V_(pp)=V_(max)−V_(mm), shown in FIG. 5. Once thestimulation threshold (I_(Thresh)) is reached, the evoked response isreproducible and increases with increasing stimulation until saturationis reached as shown in FIG. 6. This is known as a “recruitment curve.”In one embodiment, a significant EMG response is defined as having aV_(pp) of approximately 100 uV. The lowest stimulation signal current,I_(stim) that evokes this threshold voltage (V_(Thresh)) is calledI_(Thresh). Finding I_(thresh) is useful in making neurophysiologicassessments because it provides a relative indication as to the degreeof communication between a stimulation signal and nerve tissue. Forexample, as the degree of electrical communication between a stimulationsignal and a nerve decreases, I_(thresh) will increase. Conversely, asthe degree of communication between the stimulation signal and a nerveincreases, I_(thresh) will decrease.

The neuromonitoring system 10 capitalizes on and enhances theinformation derived from I_(thresh) by (a) employing methods designed tofind I_(thresh) quickly, accurately, and efficiently; (b) analyzingI_(thresh) according to predetermined safety indicator levels; and (c)displaying I_(thresh) and related safety indication data in a simple andmeaningful way. Armed with the useful information conveyed by the system10, the surgeon may detect early on any problem or potential problem andthen act to avoid and/or mitigate the situation. By way of generalexample only, an excessively high I_(thresh) or an increase over aprevious I_(thresh) measurement during Nerve Mesh Retractor mode mayindicate a deterioration of nerve root function caused by excessiveand/or prolonged retraction. During Screw Test and Detection modes, alow I_(thresh) value may indicate a breach in the pedicle, or the closeproximity of a nerve, respectively.

To quickly determine I_(thresh), the system 10 may employ a variety ofsuitable algorithms and techniques which are described in detail in the“NeuroVision Applications,” all of which are incorporated by referencebelow, as if they were set forth herein in their entireties. Oneexemplary threshold hunting algorithm, illustrated by way of exampleonly in FIGS. 7A-7D, is described hereafter in only brief detail. Thethreshold hunting algorithm utilizes a bracketing method and a bisectionmethod to find I_(thresh). The bracketing method finds a range (bracket)of stimulation currents that must contain I_(thresh). To accomplishthis, the algorithm directs stimulation to begin at a predeterminedcurrent level (based on the selected function). For each subsequentstimulation, the current level is doubled from the previous currentlevel. This doubling continues until a until a stimulation currentrecruits, that is, results in an EMG response with a V_(pp) greater orequal to V_(thresh) (e.g. 100 uV). This first stimulation current torecruit, together with the last stimulation current to have notrecruited, forms the initial bracket. If the stimulation currentthreshold, I_(thresh), of a channel exceeds a maximum stimulationcurrent, that threshold is considered out of range.

After the bracket containing the threshold current I_(thresh) has beendetermined, the initial bracket is successively reduced via thebisection method to a predetermined width. This is accomplished byapplying a first bisection stimulation current that bisects (i.e. formsthe midpoint of) the initial bracket. If this first bisectionstimulation current recruits, the bracket is reduced to the lower halfof the initial bracket. If this first bisection stimulation current doesnot recruit, the bracket is reduced to the upper half of the initialbracket. This process is continued for each successive bracket untilI_(thresh) is bracketed by stimulation currents separated by thepredetermined width. In one embodiment, the midpoint of this finalbracket may be defined as I_(thresh); however, any value falling withinthe final bracket may be selected as I_(thresh) without departing fromthe scope of the present invention.

During some functions (e.g. Screw Tests and Detection) stimulations maystop after I_(thresh) is determined for the channel possessing thelowest I_(thresh). For other functions (e.g. Nerve Retractor), however,it may useful to determine I_(thresh) for every channel. To accomplishthis quickly, the hunting algorithm may employ additional methodsallowing it to omit certain stimulations, thereby reducing the number ofstimulations and time required to obtain an Mesh value on each channel.As demonstrated in FIG. 8 and FIGS. 9A-9C, I_(thresh) is still foundusing the bracketing and bisection methods described above, however thealgorithm will omit stimulations for which the result is predictablefrom data previously acquired. When a stimulation signal is omitted, thealgorithm proceeds as if the stimulation had taken place. This permitsthe algorithm to proceed to the next required stimulation immediately,without a time delay inherently associated with each stimulation signal.To further reduce the number of stimulations required over the timeframe of an entire surgical procedure, the algorithm may confirmpreviously obtained I_(thresh) values (e.g. by stimulation at currentlevels just below and at/or just above I_(thresh) and determiningwhether the resulting responses are consistent with the previouslyacquired I_(thresh) value), rather than initiating stimulations from thebeginning each time a function is performed.

By way of example only, the various functional modes of theneuromonitoring system 10 may include the Basic Screw Test, DifferenceScrew Test, Dynamic Screw Test, MaXcess® Detection, and Free-run EMG,all of which will be described briefly below. The Basic Screw Test,Difference Screw Test, and Dynamic Screw Test modes are designed toassess the integrity of bone (e.g. pedicle) during all aspects of pilothole formation (e.g., via an awl), pilot hole preparation (e.g. via atap), and screw introduction (during and after). These modes aredescribed in greater detail in commonly owned U.S. Pat. No. 7,664,544entitled “System and Methods for Performing Percutaneous PedicleIntegrity Assessments” and commonly owned U.S. Pat. No. 7,657,308,entitled “System and Methods for Performing Dynamic Pedicle IntegrityAssessments,” the entire contents of which are both hereby incorporatedby reference as if set forth fully herein. The MaXcess® Detection modeis designed to detect the presence of nerves during the use of thevarious surgical access instruments of the neuromonitoring system 10,including the k-wire 62, dilator 64, cannula 66, 68, retractor assembly70. This mode is described in greater detail within commonly owned U.S.Pat. No. 8,068,912 entitled “System and Methods for Determining NerveProximity, Direction, and Pathology During Surgery,” the entire contentsof which is hereby incorporated by reference as if set forth fullyherein. Although not described herein, various other functional modesmay be performed by the system 10, such as for example only, MEP andSSEP functions which are described in detail within commonly owned andco-pending Int'l Patent App. No. PCT/US2006/003966, entitled “System andMethods for Performing Neurophysiologic Assessments During SpineSurgery,” filed on Feb. 2, 2006, the entire contents of which are herebyincorporated by reference as if set forth fully herein; The Twitch Testmode which is described in detail in commonly owned U.S. Pat. No.8,538,539 entitled “System and Methods for Assessing the NeuromuscularPathway Prior to Nerve Testing,” the entire contents of which is herebyincorporated by reference as if set forth fully herein; and NerveRetractor mode which is described in greater detail within commonlyowned U.S. Pat. No. 7,522,953 entitled “System and Methods forPerforming Surgical Procedures and Assessments,” the entire contents ofwhich are hereby incorporated by reference as if set forth fully herein.

In one embodiment one or more of the surgical accessories 24 including,but not necessarily limited to screw test probe 30, tap member 32, boneawl 34, k-wire 36, dilating cannulae 38, 40, retractor assembly 42, byway of fixed or releasable linkage to a stimulation handpiece 28.Turning to FIG. 10, there is shown one exemplary embodiment of astimulation handpiece 28. Stimulation handpiece 28 is communicativelylinked to the patient module 14 via accessory cable 26. Stimulationhandpiece 28 directs stimulation signals from the patient module 14 tothe surgical accessories 24. Thereafter the stimulation signalpreferably exits one or more electrode regions formed at or near thedistal ends of the surgical accessories 24. Stimulation handpiece 28 maybe equipped with one or more stimulation buttons 52 for selectivelyapplying electrical stimulation to the attached surgical accessory 24(according to the selected mode and hunting algorithm discussed above).The one or more stimulation buttons 52 are preferably positioned alongstimulation handpiece 28 such that the one or more buttons 52 maypreferably be manipulated using the thumb and/or one or more fingers ofthe hand in which handpiece 28 is held. Various surgical accessories 24that may be coupled to the stimulation handpiece 28 will be discussed inmore detail with regard to the neurophysiologic assessment modesperformed by the neuromonitoring system 10.

The neuromonitoring system 10 may test the integrity of pedicle holes(during and/or after formation) and/or screws (during and/or afterintroduction) via the Basic Screw test, Difference Screw Test, and/orDynamic Screw Test modes. For the Basic Screw Test a screw test probe30, such as that illustrated in FIG. 11, is used to direct stimulationsignals to a pilot hole prior to screw installation, or a screw headafter screw installation. Screw test probe 30 may be coupled tostimulation handpiece 28 and includes an elongated probe member 54 and aball-tipped end 56. The ball-tipped end 56 is inserted through thesurgical corridor to the stimulation target site (e.g. pilot hole and orscrew head). Once the ball-tipped end 56 is in position, a stimulationbutton 52 may be pressed to initiate stimulation. The insulatingcharacter of bone will prevent the stimulation current, up to a certainamplitude, from communicating with the nerve, thus resulting in arelatively high I_(thresh), as determined via the threshold huntingalgorithm described above. However, in the event the pedicle wall hasbeen breached by the screw or tap, the current density in the breacharea will increase to the point that the stimulation current will passthrough to the adjacent nerve roots and they will depolarize at a lowerstimulation current, thus I_(thresh) will be relatively low. Details andresults of the Basic Screw test results and may be conveyed to the useron display 46.

In Difference Screw Test mode, a baseline threshold value is determinedby directly stimulating a nerve. Screw test probe 30 may preferably beused, and the probe is advanced through the surgical corridor to thesurgical target site (i.e. the nerve to be directly stimulated). Button52 on the stimulation handpiece 28 is pressed to initiate stimulationand a baseline threshold is established. Screw test probe 30 may then bemaneuvered to the next stimulation target site (e.g. pilot hole or screwhead) and stimulation is initiated to determine the actual thresholdvalue I_(thresh). The actual threshold is compared to the baselinethreshold. The difference between the actual and baseline thresholds iscalculated to provide an indication of the safety level. Details andresults, including the baseline, actual, and difference thresholds amongother things may be displayed for the user on GUI display 46.

Dynamic Screw Test mode continuously monitors threshold values while oneor more surgical accessories are in use, for example forming a pilothole. For dynamic screw tests an electric coupling device, such as, byway of example only, stimulation clip 58 is coupled to stimulationhandpiece 28, as illustrated in FIG. 12. The electric coupling devicecouples surgical accessories 24 (such as for example, a tap member 32 ora bone awl 34) to the neuromonitoring system 10 such that stimulationsignals may be transmitted through the tool during use. Thus, screwtesting may be performed continuously during pilot hole formation bycoupling the bone awl 34 to the neuromonitoring system 10, and duringpilot hole preparation by coupling the tap 32 to the system 10.Likewise, by coupling a pedicle screw to the neuromonitoring system 10(such as via pedicle screw instrumentation), screw testing may beperformed during screw introduction. To continually update theI_(thresh) results in an efficient manner, the algorithm may preferablyconfirm the earlier results by switching back and forth betweenstimulation signals just above and just below I_(thresh). If theexpected results are not obtained then the algorithm may transition backinto the bracketing and bisection steps. Details and results of theDynamic Screw test results and may be conveyed and continuously updatedon display 46.

With reference to FIG. 1 and FIG. 12, stimulation clip 58 utilizes aspring-loaded plunger to hold the surgical tool and transmit thestimulation signal. The plunger 60 is composed of a conductive materialsuch as metal. A nonconductive housing 62 partially encases the plunger60 about its center. Extending from the housing 62 is an end plate 64.An electrical cable 66 connects the stimulation clip 58 to thestimulation handpiece 28. A spring (not shown) is disposed within thehousing 62 such that in a natural or “closed” state the plunger 60 issituated in close proximity to the endplate 64. Exerting a compressiveforce on the spring (such as by pulling the cable 66 while holding thehousing 62) causes a gap between the end plate 64 and the plunger 60 towiden to an “open” position, thereby allowing insertion of a surgicaltool (e.g. tap member 32 and awl 34) between the end plate 64 andplunger 60. Releasing the cable 66 allows the spring to return to a“closed” position, causing the plunger 60 to move laterally back towardsthe endplate such that a force is exerted upon the surgical instrumentand thereby holds it in place between the endplate 64 and the plunger60. This is best viewed in FIG. 13 wherein stimulation clip 58 is linkedto stimulation handpiece 28 (via cable 66) on one end and coupled aroundtap member 32 on the other end. Alternatively, stimulation clip 58 (orclap 68 described below) may be linked directly to patient module 14rather than stimulation handpiece 28, in which case stimulation may beinitiated and/or stopped from the GUI display 46. The electricalstimulus may be initiated by pressing one of stimulation buttons 52 andthereafter the stimulation signal may be passed from the handpiece 28through the cable 66 and plunger 60 to the tap member 32 (or othersurgical accessory 24).

Again with reference to FIG. 1, there is shown another embodiment of anelectric coupling device for use with the system 10. Stimulation clamp68 is comprised of two prongs hingedly coupled at a coupling point 70such that the clamp 68 includes an attachment end 72 and anon-attachment end 74. A stimulation electrode 76 is disposed on theattachment end 72 and communicates with electric cable 66 extending fromthe non-attachment end 74 to the handpiece 28. In a “closed” positionthe prong ends at the attachment end 72 touch. Depressing the prongs atthe non-attachment end 74 in a direction towards one another causes agap to form between the prong ends at the attachment end 72. Positioningthe “opened” attachment end 72 over a desired surgical instrument andreleasing the force on the non-attachment end 74 causes the attachmentend 72 to pinch tight on the surgical accessory 24 and thereby allow theelectrical stimulus to pass from the stimulation handpiece 28, throughthe stimulation electrode 76, to the surgical accessory.

Stimulation results and other relevant data for the screw test modes areconveyed to the user on display 46, as illustrated in FIGS. 14-16. FIG.14 is an exemplary screen view of the Basic Screw Test mode for displayon display 46. FIG. 15 illustrates an exemplary screen view of theDifference Screw Test mode for display on display 46. FIG. 16 is anexemplary screen view of the Dynamic Screw Test mode for display ondisplay 46. Upon execution of the algorithm, one or more channel tabsmay be highlighted using a color-code to indicate status of thecorresponding nerve, and thus the relative safety level determined bythe system 10. The channel with the “worst” (lowest) level willpreferably be enlarged and that myotome name 76 will be displayed, aswell as graphically depicted on the spine diagram 78. A vertical barchart 80 may also be shown to depict the stimulation current required toevoke a significant response for the selected channel. A large numericalreadout 82 may also indicate the value of the stimulation result.Preferably, the display of the stimulation result may be augmented witha color code utilizing the colors green, yellow, and red to enhance theunderstandability of the result and quickly indicate to the surgeon thelevel of safety determined by the system 10. Red may be used to indicatean I_(thresh) level below a predetermined unsafe level. Yellow may beused to indicate an I_(thresh) that falls in between predetermined safeand unsafe levels. Green may represent an I_(thresh) within the rangepredetermined as safe. Although not show, the threshold results may bereplaced with, or more preferably, augmented with a display of theactual waveform for each channel, as well as audible sounds distinctiveto each level of safety (safe, unsafe, in between).

The neuromonitoring system 10 may perform nerve proximity testing, viathe MaXcess® Detection mode, to ensure safe and reproducible access tosurgical target sites. Using the surgical access components 36-40, thesystem 10 detects the existence of neural structures before, during, andafter the establishment of an operative corridor through (or near) anyof a variety of tissues having such neural structures, which, ifcontacted or impinged, may otherwise result in neural impairment for thepatient. The surgical access components 36-40 are designed to bluntlydissect the tissue between the patient's skin and the surgical targetsite. Access components 36-40 preferably utilize stimulation handpiece28 and stimulation clip 58 (in the same manner as described above andshown in FIG. 12) to link to the system 10. Cannulae or dilators ofincreasing diameter may be advanced towards the target site until asufficient operating corridor is established. As the cannulae ordilators are advanced to the target site, electrical stimulation signalsare transmitted through the stimulation handpiece 28 to the distal endof the cannulae where they are emitted from an electrode region. Thestimulation signal will stimulate nerves in close proximity to thestimulation electrode and the corresponding EMG response is monitored.As a nerve gets closer to the stimulation electrode, the stimulationcurrent (I_(stim)) required to evoke a muscle response decreases.I_(thresh) is calculated (using the threshold hunting algorithmdescribed above) which provides a measure of the communication betweenthe stimulation signal and the nerve and thus giving a relativeindication of the proximity between access components and nerves.

Additional and/or alternative surgical access components such as, by wayof example only, a tissue retraction assembly 42 (FIG. 1) may be coupledto the system 10 (via stimulation clip 58 or clamp 68) and employed toprovide safe and reproducible access to a surgical target site. Tissueretraction assembly 42 and various embodiments and uses thereof havebeen shown and described co-pending and commonly assigned U.S. patentapplication Ser. No. 10/967,668, entitled “Surgical Access System andRelated Methods,” filed on Oct. 18, 2004, the entire contents of whichare expressly incorporated by reference as if set forth herein in theirentirety.

An exemplary screen display of the Detection mode for display on display46 is illustrated by way of example only in FIG. 17. Similar to thescrew test modes, upon execution of the algorithm, one or more channeltabs may be highlighted using a color-code to indicate status of thecorresponding nerve, and thus the relative safety level determined bythe system 10. The channel with the “worst” (lowest) level willpreferably be enlarged and that myotome name 76 will be displayed, aswell as graphically depicted on the spine diagram 78. A vertical barchart 80 may also be shown to depict the stimulation current required toevoke a significant response for the selected channel. A large numericalreadout 82 may also indicate the value of the stimulation result.Preferably, the display of the stimulation result may be augmented witha color code utilizing the colors green, yellow, and red to enhance theunderstandability of the result and quickly indicate to the surgeon thelevel of safety determined by the system 10. Red may be used to indicatean I_(thresh) level below a predetermined unsafe level. Yellow may beused to indicate an I_(thresh) that falls in between predetermined safeand unsafe levels. Green may represent an I_(thresh) within the rangepredetermined as safe. Although not show, the threshold results may bereplaced with, or more preferably, augmented with a display of theactual waveform for each channel, as well as audible sounds distinctiveto each level of safety (safe, unsafe, in between).

The neuromonitoring system 10 may also conduct free-run EMG monitoringwhile the system is in any of the above-described modes. Free-run EMGmonitoring continuously listens for spontaneous muscle activity that maybe indicative of potential danger. The system 10 may automatically cycleinto free-run monitoring after 5 seconds (by way of example only) ofinactivity. Initiating a stimulation signal in the selected mode willinterrupt the free-run monitoring until the system 10 has again beeninactive for five seconds at which time the free-run begins again.Stimulated and/or Free-run results for any function may be replacedwith, or more preferably, augmented with a display of the actualwaveform for each channel, as well as audible sounds distinctive to eachlevel of safety (safe, unsafe, in between).

To augment the neurophysiologic assessments, such as for example onlythose described above, performed by the neuromonitoring system 10, thesystem 10 may be further equipped to conduct and display ultrasoundimaging of proximate body tissues (e.g. bone during pilot hole formationand preparation and/or screw implantation and nerves and/or vasculatureduring surgical access). To do so, the system 10 may employintraoperative ultrasound tailored to allow use within bone, such as, byway of example only, the ultrasound system described in U.S. Pat. No.6,579,244, entitled “Intraosteal Ultrasound During SurgicalImplantation.” Specifically, at least one ultrasound transducer 55 maybe deployed to the surgical target site during surgery. Under thedirection of control unit 12, acoustic signals of a predeterminedfrequency, ranging between 50 kHz and 16 MHz, are emitted from thetransducer(s) 55 through the surrounding body tissue. The signalsreflect off tissue boundaries and are thereafter received back at thetransducer, converted into electric signals, and processed by thecontrol unit 12 into viewable images. The images may be viewed on thescreen display 26.

Preferably, at least one transducer is mounted on or within one or moreof the surgical accessories 24 (such as screw test probe 30, dilatingcannula 38, 40, or retraction assembly 42, shown in FIG. 1) and/or oneor more surgical instruments engageable with the system 10 via electriccoupling device 58, 68 (such as a tap member 34 or pedicle access probe32, shown in FIG. 18 and FIG. 19, respectively). As an alternative, aseparate transducer (not shown) may be provided and advanced to thesurgical target site alone or in conjunction with one or more of theabove accessories or instruments, either by advancing alongside orthrough an interior lumen formed in the instrument for such purpose.Incorporating the transducer 55 onto existing instrumentation permitsdeployment to the surgical target site during normal operation of theinstrument, allowing the advantageous addition of ultrasound imagingwithout requiring additional steps or instrumentation. It is furthercontemplated that ultrasound transducers 55 may be deployed at thedistal end of various bone screws, including, but not necessarilylimited to pedicle screws and/or facet screws, for enabling ultrasoundimaging of the surrounding bone during screw implantation.

For the purposes of example only, FIG. 20 depicts in more detail theincorporation of the ultrasound transducer 55 within tap member 34,according to one exemplary embodiment of the present invention. In theexemplary embodiment, ultrasound transducer 55 is incorporated in thedistal end of tap member 34. The transducer 55 may be coupled to thepatient module via electric cable, such as by way of example, cable 59.In the pictured embodiment, the surgical system 10 utilizes a 64-elementarray transducer, such as that available and in use with a number ofcommercially developed products from Volcano Corp. (Rancho Cordova,Calif.). As best appreciated in FIG. 21, the transducer elements 57 arearrayed in a circular pattern providing for 360° radial imaging ofsurrounding tissue. Optionally, forward looking transducers may also beemployed for additional imaging of tissue lying in front of the surgicalinstrument. It will be appreciated from the above description thattransducer 55 both transmits and receives the acoustic signals. Variousother configurations for integrating ultrasound capabilities are alsopossible. By way of example only, a transducer incorporated in asurgical accessory may transmit acoustic signals to a receiverpositioned in the operating room (outside the patients body) andconversely, a transducer positioned in the operating room may transmitacoustic signals to a receiver incorporated into a surgical accessory.

A basic principle underlying the effective use of ultrasound duringand/or after pilot hole formation and preparation is the distinctiveacoustical characteristics of bone relative to other soft tissues in thebody, and more importantly, the varying acoustical characteristicsexhibited by bone itself, depending upon its different properties, suchas (by way of example only) the type of bone (i.e. cortical orcancellous), bone density, and bone composition. Different acousticalcharacteristics can include, among others, the velocity, amplitude, andattenuation of sound waves as they pass through tissue. Methods aboundin the prior art for quantifying different properties of bone by usingultrasound to determine one or more of its acoustical characteristicsand additional methods are known in the prior art for processingultrasound signals to generate a viewable image of tissue. The presentinvention makes advantageous use of this information, as well as thegeneral makeup of the boney tissue within the pedicle, to assistsurgeons in guidance of surgical instrumentation (including but notlimited to tap member 34 and pedicle access probe/awl 36) through thecancellous bone of the interior pedicle and into the vertebral bodywithout breaching the cortical wall.

With reference to FIG. 22, the pedicle generally comprises a hard outerregion 420 of dense cortical bone and center region 400 of softer, lessdense cancellous bone, separated by one or more middle regions 410. Thebone in region 410 is generally less dense than outer region 420 butmore dense than center region 410. The preferred trajectory for pediclescrew placement is through the soft cancellous bone of the center region400, thereby avoiding a breach of the pedicle wall. During and/or afterpilot hole formation and preparation at least one of the tap member 34,pedicle access probe 36, or screw test probe 30, is advanced through thepedicle. As illustrated in FIG. 23, acoustic signals are emitted fromthe transducer 55 and travel through the bone. Upon reaching a boundary,a portion of the signal reflects back to the transducer while theremainder of the signal continues moving through the pedicle to the nextboundary. The received signals are electronically processed andconverted into an image that graphically represents the differenttissue, specifically, the different regions of bone 400, 410, and 420.In a preferred embodiment, the ultrasound scan is conducted radiallyabout the surgical instrument thus generating a 360° image of thepedicle relative to the distal end of the surgical instrument, as seenin FIG. 24. From the screen display 46 the surgeon can visually monitorthe relative position of the instrument within the pedicle and thus makeany necessary adjustments should the instrument position stray from thedesired pathway. In another embodiment, the transducer 55 may besubstantially located on just one portion of the distal end and theultrasound scan may be directed by rotating the instrument about itslongitudinal axis or any desired portion thereof.

In addition to the image guidance aspect of ultrasound, ultrasound maybe used to determine various properties and/or conditions of bone (viaany of a number of suitable methods known to the prior art which may beimplemented by the system 10) which may also provide useful information.By way of example only, cracks in the pedicle bone, along with theirrelative position, may be detected using ultrasound. The system 10 maythus detect a breach in the outer wall of the pedicle by ultrasounddetection as well as by the nerve monitoring described above. Additionalwarning indicia such as graphics and/or audible tones may be employed towarn of any danger detected by the system 10 using ultrasound. By way offurther example, the system 10 may utilize ultrasound to determine thedensity of the bone instrument 24 is in contact with. In the event theinstrument encounters cortical bone an auditory or visual alert may beinitiated thereby providing additional warning of impending breach ifthe current trajectory is maintained.

Ultrasound during surgical access may also be used to enhance the nervedetection function described above and proceeds along the same premiseas that described for imaging bone. Acoustic signals, generally in therange of 2 MHz-16 MHZ for nerve imaging, are emitted from the transducer55 located on or within the surgical access components (such as,cannulae 38, 40, and/or retraction assembly 42). The signals reflect oftissue boundaries, such as the interface between fat and muscle ormuscle and nerves, and are thereafter received and processed to form aviewable image of the tissue relative to the transducer, which isdisplayed on screen display 46. Nerves are distinguished from othertissue based on their shape and/or color on the image, as illustrated inFIG. 25. By way of example only, nerves generally appear as round oroval shaped and are generally brighter than the surrounding tissue.

Ultrasound may be utilized on system 10 in conjunction with one theneurophysiologic assessment functions, or, it may be used as a standalone feature. In one embodiment ultrasound is preferably activated fromthe GUI display 46 by selecting the appropriate command. When ultrasoundimaging is utilized in conjunction with nerve monitoring according tothe present invention, the ultrasound image is preferably displayedtogether with the nerve monitoring data thereby allowing the user toreceive all the useful information provided by the system 10 at one timewithout the need to switch between screen views. FIG. 26-28—illustrate,by way of example only, an exemplary screen display for the Basic,Difference, and Dynamic screw test modes, respectively, with thecombined ultrasound imaging image 84. In one embodiment, the ultrasounddisplay includes colorized tissue boundaries. The soft cancellous bone400 may be shown in green representing the preferred instrumentpositioning. The middle region 410 may be represented in yellow and thecortical bone in red. The distal end of the surgical instrument isrepresented, by way of example only, as a target finder 430. FIG. 29illustrates an exemplary screen display of the nerve detection functionwhen ultrasound imaging is in use.

It may also be advantageous for neurophysiologic assessment data and/orultrasound images captured by the system 10 to be viewable by personsnot present in the operating room. It is contemplated that the data andimages may be transmitted to one or more remote locations and viewableby authorized persons. This may be accomplished by any number of datatransmission methods. In one example, the images may be transmitted to aremote user via remote monitoring software such as that described indetail in the commonly owned and co-pending U.S. patent application Ser.No. 11/418,589, entitled “System and Methods for Performing andMonitoring Neurophysiologic Assessments,” filed on May 5, 2006, theentire contents of which are incorporated by reference herein as if setforth in its entirety.

While this invention has been described in terms of a best mode forachieving this invention's objectives, it will be appreciated by thoseskilled in the art that variations may be accomplished in view of theseteachings without deviating from the spirit or scope of the presentinvention. By way of example the present invention may be implementedusing any combination of computer programming software, firmware orhardware. As a preparatory step to practicing the invention orconstructing an apparatus according to the invention, the computerprogramming code (whether software or firmware) according to theinvention will typically be stored in one or more machine readablestorage mediums such as fixed (hard) drives, diskettes, optical disks,magnetic tape, semiconductor memories such as ROMs, PROMs, etc., therebymaking an article of manufacture in accordance with the invention. Thearticle of manufacture containing the computer programming code is usedby either executing the code directly from the storage device, bycopying the code from the storage device into another storage devicesuch as a hard disk, RAM, etc. or by transmitting the code on a networkfor remote execution. As can be envisioned by one of skill in the art,many different combinations of the above may be used and accordingly thepresent invention is not limited by the specified scope.

1. A system for minimally invasive surgery, the system comprising: aretractor assembly including a retractor body and at least two retractorblades extending generally perpendicularly to the retractor body, theretractor body being operable to separate the at least two retractorblades relative to each other to retract tissue away from an interior ofthe at least two retractor blades when the retractor blades are advancedto a surgical target site; and a means for emitting ultrasound signalsproximate the at least two retractor blades. 2-20. (canceled)
 21. Amethod for avoiding nerve tissue during access to a surgical target siteat the spine of a patient, comprising: advancing a surgical instrumentthrough non-bone soft tissue to the surgical target site, wherein thesurgical instrument includes a retractor assembly and is equipped with ameans for emitting ultrasound signals; and during the advancing of thesurgical instrument: emitting ultrasound signals proximate a distal endof the surgical instrument; processing the ultrasound signals to monitorfor at least one of nerve tissue and vasculature located proximate thesurgical instrument; indicating on a display a position of the at leastone of nerve tissue and vasculature relative to the surgical instrument;and determining whether to adjust a path of the surgical instrument tothe surgical target site if at least one nerve tissue and vasculature ispositioned within the path to the surgical target site.
 22. The methodof claim 21, wherein the surgical target site is includes at least oneof intervertebral disc and a vertebral body.
 23. The method of claim 21,wherein the advancing of the surgical instrument through non-bone softtissue to the surgical target site includes advancing the surgicalinstrument to the surgical target site via a lateral, trans-psoas pathfrom an incision site to the surgical target site.
 24. The method ofclaim 21, further comprising: processing ultrasound signal data intoviewable images for viewing on the display.
 25. The method of claim 21,wherein the emitting of the ultrasound signals includes emitting theultrasound signals circumferentially about the distal end of thesurgical instrument.
 26. The method of claim 21, further comprising:displaying a nerve tissue on the display in a first color or first shadeof color distinct from a second color or second shade of color used todesignate surrounding tissue.
 27. The method of claim 21, furthercomprising: adjusting the path of the surgical instrument to thesurgical target site upon the determination that at least one nervetissue or vasculature is within the path.
 28. The method of claim 21,further comprising: receiving the ultrasound signals after theultrasound signals reflect off the at least one of the nerve tissue andvasculature proximate the medical accessory.
 29. The method of claim 21,wherein the retractor assembly includes at least two retractor blades.30. The method of claim 29, wherein the at least two retractor blades isthree retractor blades.
 31. A method for guiding surgicalinstrumentation through anatomy of a subject, the method comprising:creating a surgical access corridor by advancing a surgical instrumentalong a lateral, trans-psoas path from an incision site through non-bonesoft tissue to a surgical target site at an anterior column of thespine, wherein the surgical instrument is a retractor assembly andincludes a means for emitting ultrasound signals; and while creating thesurgical access corridor: emitting ultrasound signals proximate thesurgical instrument; processing the ultrasound signals to monitor for atleast one of nerve tissue and vasculature located proximate the surgicalinstrument; indicating on a display a position of the at least one ofnerve tissue and vasculature relative to the surgical instrument; anddetermining whether to adjust the path of the surgical instrument if atleast one nerve tissue and vasculature is positioned within the path tothe surgical target site.
 32. The method of claim 31, furthercomprising: while creating the surgical access corridor, processingultrasound signal data into viewable images for viewing on the display.33. The method of claim 31, wherein the emitting of the ultrasoundsignals includes emitting the ultrasound signals circumferentially abouta distal end of the medical accessory.
 34. The method of claim 31,further comprising: adjusting the path of the medical accessory to thesurgical target site upon the determination that at least one nervetissue or vasculature is within the path.
 35. The method of claim 31,further comprising: receiving the ultrasound signals after theultrasound signals reflect off of the at least one of the nerve tissueand vasculature proximate the medical accessory.
 36. The method of claim31, wherein the retractor assembly includes at least two retractorblades.
 37. The method of claim 36, wherein the at least two retractorblades is three retractor blades.